The field of the invention is variable speed induction motor drives, and particularly, closed-loop, inverter drives for operating induction motors over a wide speed range under varying load conditions.
The speed of an induction motor is controlled by varying the frequency of the power applied to its stator windings. In order to obtain substantially full-load torque capability at all operating speeds it is also necessary to be able to achieve maximum air-gap flux in the motor. In a voltage-source inverter drive, flux is often held near maximum at all times by maintaining a constant motor voltage-to-motor speed ratio, and in a current-source inverter drive, constant flux can be accomplished by maintaining a functional relation between stator current and motor slip speed. Although such control strategies can be simply stated, their economical implementation and efficiencies for many control applications has to date been lacking.
Numerous closed loop control strategies of varying complexity have been proposed to control induction motors. Most of these require the sensing of motor speed, which in turn requires the use of a shaft speed sensor. Such speed sensors add cost and decrease the reliability of the system, at least in the context of industrial motor drives. More recent strategies such as that described in "Inverter Fed Induction Motor Drive Using Power Factor Control" published in the Journal of the 1976 annual meeting of the IEEE Industry Application Society by S. A. Rosenberg, S. B. Dewan and G. R. Slemon avoid the use of tachometers, but a control strategy which relies soley on power factor control is not desirable. For any given power factor, for example, there are two possible electrical frequencies for any given motor speed and the maintenance of one stable solution is not easy under a wide range of operating conditions. Indeed, it is often desirable to operate a motor at or near a maximum power factor point which would not be desirable as a power factor control circuit set point. This is because circuit component values which are out of tolerance could cause unrealizable power factor requirements which may result in an undesirable high slip frequency operation with consequent poor efficiency and torque capability.